I don't know what spark plugs you would need, but $300 is definitely ridiculous. Also, there's a recall for our cars of the coil packs. When one fails, that also causes symptoms like you experienced. If it was a coilpack failure, it should be replaced for free.
2007 A4 2.0T CVT Cabriolet S-Line Phantom Black
APR 93, K&N air filter, Podi boost gauge, chrome-stripped grille, RS4 RSB
Smoked reflectors & markers, smartTOP, Eibach Pro-Kit, 12F/15R spacers, ER SMIC
Xenon match DRLs, HID fogs, S4 mirrors, red LED interior, CC, 034 HFC + Magnaflow 14829